The present invention relates to silalactones, to a method for their preparation and to methods for preparing carboxyalkyl-substituted organopolysiloxanes therefrom.
Carboxyalkyl-substituted organopolysiloxanes are known from U.S. Pat. Nos. 2,723,987; 2,900,363; 3,119,855 and 3,391,177 and their utility as metal protectants and paper sizings is known from U.S. Pat. Nos. 3,755,071 and 4,011,362.
The methods disclosed in the art for the preparation of carboxyalkyl-substituted organopolysiloxanes typically comprise a hydrolysis reaction of a cyanolkyl-substituted hydrolyzable silane or a carbalkoxyalkyl-substituted hydrolyzable silane to prepare the carboxyalkyl-substituted siloxane unit, followed by a silanol condensation reaction and/or a siloxane equilibration reaction to provide the desired carboxyalkyl-substituted siloxane.
However, these methods for preparing carboxyalkyl-substituted organopolysiloxanes are not completely satisfactory in-as-much as the hydrolysis reaction of said cyano- or carbalkoxy-alkyl substituent is rarely complete and the final organopolysiloxane contains various amounts of residual radicals, such as cyanoalkyl radicals or carbalkoxyalkyl radicals. A method for preparing carboxyalkyl-substituted organopolysiloxanes which are free of such residual radicals is desired.
Silalactones are known from U.S. Pat. Nos. 2,589,446; 2,635,109; 2,963,500 and 3,395,167; however, the silalactones disclosed therein contain, or give rise to, triorganosiloxy units and are therefore not useful for preparing organopolysiloxanes which contain more than two carboxyalkly substituents per molecule. Although the above-noted silalactone patents disclose that the triorganosiloxy-containing silalactones described therein are useful for preparing disiloxane dicarboxylic acids and various organofunctional organosiloxanes, no further teachings relative to carboxyalkyl-substituted organopolysiloxanes are given. Examples of said triorganosiloxy-containing or -forming silalactones of the art include ##STR2## wherein Me denotes the methyl radical.
U.S. Pat. No. 3,395,167 further discloses a process for preparing the triorganosiloxy-containing silalactone. Said process comprises heating an ester having the formula XR.sub.2 Si(CR.sub.2 ').sub.n COOA. The use of a halide salt catalyst to aid the reaction or the use of an ester containing two silicon-bonded X atoms was not contemplated in said patent.
U.S. Pat. No. 4,329,483 discloses a process for preparing a cyclotetrasiloxane and either an acyl chloride or an aliphatic chloride. Said process comprises sufficiently heating a reaction mass comprising an ester group and at least one .tbd.SiCl group until the cyclotetrasiloxane is formed. Halide salt catalysts were said to aid the reaction. Although some of the reaction mixtures that are used in the method of the present invention were disclosed as a suitable reaction mass, the preparation of silalactones was not disclosed.